The physical size of AC adapters and chargers for portable devices such as laptop and smartphones is an important consideration in system design. Two specific constraints define the minimum physical size of conventional adapters: the physical volumetric size consumption of the components making up the adapter; and the thermal consideration, specifically maximum skin temperature for e.g. plastic enclosures determines a maximum internal power dissipation for a given surface area (assuming uniform power dissipation/heat distribution inside the enclosure). Both constraints taken together yields a minimum possible physical size limitation using state-of-the art approaches. More specifically, the state-of-the-art approach can be described as a combination of a ubiquitous input stage combined with a converter stage.
The converter stage is traditionally implemented using some form of the flyback topology, but in some cases, also forward or similar topologies, which are essentially wide input voltage range capable DC/DC converters providing galvanic isolation (and typically voltage step-down by means of a transformer, which also provides galvanic isolation). The input stage for AC adapters operating below 65 W and with no PFC (power factor correction) requirement typically has input protection circuitry, EMI (electromagnetic interference) filter, a diode bridge rectifier and a bulk-capacitor for AC line frequency filtering.
Conventional input stages require significant physical volumetric space consumption as well as significant power losses, none of which are addressable by typically targeted loss mechanisms or space consumers in the converter. For example, Even if an ideal converter stage with no volumetric space consumption and zero power dissipation were realizable, the input stage still imposes an upper limit on power density, which is not too far away from best-in-class demonstrated power densities achieved to date (approximately 15 W/in3 can be achieved in a 65 W converter using this approach, but about 50% of the volumetric space consumption and at least 30% of the power dissipation resides in the input stage, thus setting the power density of ˜15 W/in3 for 65 W as possibly the highest practically achievable power density with this approach. With an ideal converter stage, the theoretically highest power density achievable by is on the order of 30 W/in3, but of course is not practically achievable.